An excessive rainfall event, resulting from an approaching tropical storm and a stationary surface boundary, brought record rainfall and flooding to much of northeast Florida and extreme southeast Georgia June 24-27. Tropical Storm Debby brought tropical moisture across the area, while a nearly stationary surface trough of low pressure over far northern FL lifted the moist air to produce a nearly stationary band of very heavy rainfall. The surface map below illustrates the surface trough (red dashed line) over far northern FL, with southerly winds south of the trough, northeasterly winds north of the trough, resulting in strong convergence of air along the trough. This was a key feature that set up the nearly stationary rain band along and just north of Interstate 10 in far northern FL.

Surface observations for 6am EDT June 26, 2012.

Falling at rates of 1-2 inches per hour, rainfall totaled 5-10 inches across much of the area (red shades on image below), with an enhanced band of 12-20 inches along the stationary trough over far northern FL, along and just north of Interstate 10. Radar estimated storm-total rainfall, shown below, clearly shows the enhanced rainfall band over far northern FL (magenta, purple and white shades).

Radar-estimated storm total rainfall amounts, in inches, June 24-27.

The excessive rainfall led to significant urban and rural flooding, closing roads and flooding structures. The rainfall also led to major river flooding. Late Tuesday night, the Black Creek at Middleburg was approaching the forecast crest right at the record flood stage around 25 feet, as shown below (thick line observed, thin line forecast).

Observed (thick line) and forecast (thin line) river stage on the Black Creek at Middleburg.

While slow-moving tropical storms can produce very heavy rainfall, tropical systems that interact with a stationary surface boundary can produce excessive amounts and severe flooding, as was the case across northern Florida with Debby. Daily and monthly rainfall records were set at Jacksonville and Gainesville during this event. Storm event rainfall totals reached 5.97 inches at St. Simons Island GA, 12.02 inches at Gainesville (FL) regional airport, and 13.78 inches at Jacksonville (FL) International Airport.

Additional information, including other measured rainfall amounts, will be provided on the NWS Jacksonville website later.